The present invention relates to solar water heating systems; and more particularly, it relates to freeze protection for thermosiphon solar water heating systems.
There are fundamentally two types of solar water heating systems. One type is a forced circulation system that utilizes a pump to move water between the water storage tank and the solar energy collector. The other type operates by natural circulation provided by thermosiphon action. In a thermosiphon solar water heating system, a solar energy collector is coupled by colder water inlet and heated water outlet pipes to a heated water storage tank disposed near by and at a higher elevation. Water heating in the solar energy collector expands; and as it does so, it becomes more buoyant. The heated water naturally rises from the solar collector into the storage tank, and is replaced by colder water from the tank.
Thermosiphon solar water heating systems are, however, subject to freeze rupture damage upon being exposed to cold weather at times when there is little solar energy, such as at night or on cloudy days. Accordingly, thermosiphon systems have largely been limited to mild climate areas. When thermosiphon systems have been installed for use in areas where there is risk of freeze damage, freeze rupture protection has been provided by draining the system, by forcing water to circulate through the system using an auxiliary pump, by heating the system with an auxiliary heater device, or by flushing the system with fresh warm water that is subsequently discarded.